Solid-state X-Ray detectors having electronic sensors of X-Ray electromagnetic energy, rather than chemical film-based sensors. The solid-state X-Ray detectors are often referred to as electronic X-Ray detectors.
One type of conventional solid-state X-Ray detector consists of an array of pixels composed of switches as FETs (field effect transistors) and light detectors such as photodiodes, the pixels being constructed of amorphous silicon, over which Cesium Iodide (CsI) is deposited. CsI absorbs the X-rays and converts them to light, which is then detected by the photodiodes. The photodiode acts as a capacitor and will store charge. Initialization of the detector takes place prior to an X-Ray exposure, when during the course of “scrubbing” the detector, each photodiode is charged to a known voltage. The detector is then exposed to X-Rays which are absorbed by the CsI. Light that is emitted in proportion to the X-Ray flux then partially discharges the photodiode. After the conclusion of the exposure, the voltage on the photodiode is restored to the initial voltage. The amount of charge required to restore the initial voltage on the photodiode is measured, which becomes a measure of the X-Ray dose integrated by the pixel during the length of the exposure.
In accordance with the array like structure of the detector, the detector is read, or scrubbed, on a scan line by scan line basis. Reading of the detector is controlled by the FET switch associated with each photodiode. Reading is performed whenever the image produced by the detector contains valuable data. Valuable data includes images that contain exposure data and images that contain offset data. Scrubbing is very similar to reading except that the data is not informative, and is therefore discarded. Scrubbing is performed to maintain proper bias on the diodes during idle periods, or to perhaps reduce the effects of lag, which is incomplete charge restoration of the photodiodes, among other reasons. Scrubbing must restore charge, but by definition, the charge need not be measured. If the charge is measured, the data can be simply discarded.
A distinct benefit of the architecture of the solid-state X-Ray detector is that the presence of the switching element minimizes the number of electrical contacts that would need to be made to the detector. If no switching elements were present, at least one contact for each pixel would need to be present on the detector. A detector with over 1 million pixels would be impossible to develop or produce. The switching element reduces the number of required contacts to no more than the number of pixels along the perimeter of the array. The pixels in the interior of the array are “ganged” together along each axis of the array. An entire row of the array is controlled simultaneously when the scan line attached to the gates of all the FETs of pixels on that scan line is activated. Each of the pixels on that scan line is connected to a separate data line, through the switch, which is used by the read out electronics to restore the charge to the photodiode. As each scan line is activated in turn, all of the pixels in that scan line have the charge restored to the respective photodiode simultaneously by the read out electronics over the individual data lines. Each data line is associated with a dedicated read out channel.
The bias voltage to which the photodiodes are charged is simply the difference in potential between the voltage at a common contact, and the voltage of the photodiode's respective data line. In order for the photodiode to store the charge on the capacitance of the photodiode, the photodiode is reverse biased, meaning the common contact connects all of the photodiode's anodes together and is more negative in potential than any of the data lines. While the read out channel often will maintain the potential of the associated data line at what is known as a virtual ground, the read out channel may in fact be at some actual potential slightly above or below ground potential. This may be due to the architecture, implementation, or perhaps simply process variation of the read out channel design.
Each pixel in the detector, and each readout channel will have variations in gains and offset relative to other pixels and readout channels. Consequently, in order to present the best image quality, X-Ray images have these variations normalized or corrected, before the image is presented for patient diagnosis. Offset readings, requiring no X-Rays, can be taken any time, and in fact to get the most accurate offset reading, a “dark” image is often acquired close in time to the X-Ray image. As part of the correction, the “dark” image is subtracted from the X-Ray image. Gain calibration and correction, however, requires X-Ray images. Prior to the first patient, one or more “flat field” X-Ray images are acquired, with nothing between the X-Ray tube and the detector. If there were no gain, offset or X-Ray flux variations over the surface of the detector, and no gain or offset variation in the readout channels, all of the pixels would report exactly the same value. However, this is known not to be the case. This flat field X-Ray image is then used to normalize all of the individual pixels (after offset correction) to nearly the same value. The factor used to normalize each pixel then becomes a gain “map.” By default, because the readout channels are used to acquire these images, the gain and offset variations of the readout channels are also corrected by the gain map, offset correction and no special separate treatment of gain and offset variations of the readout channels are required. This gain map is then used to normalize all of the diagnostic X-Ray images that are acquired subsequently.
Detectors are composed of discrete detecting or imaging elements numbering in the millions of which at least one of the imaging elements will be defective. Recognizing that the probability of fabricating a defect free panel is very low, a special interpolating correction is performed for those pixels whose offset or gain values fall outside an acceptable range. These “bad pixels” are replaced by a combination of neighboring pixels. The bad pixels are identified in the offset image by an offset being too high (too much leakage, for example) or an offset being too low (saturated). In the gain image, the bad pixels may be saturated (high) or may not react enough to the X-Ray stimulus to be considered viable. However, this calibration also requires X-Rays.
In order to maintain optimum image quality, frequent calibration is encouraged. However, this requires user intervention, to generate the X-Rays at a time when there is nothing “in the beam” in front of the detector. During calibration, the system is not available for use, resulting in loss of productivity of the X-Ray system. Furthermore, because human intervention is required, mistakes are often made which may adversely affect image quality until the next calibration is performed.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to calibrate the gain of the solid-state X-Ray detectors without projecting an X-Ray beam onto the detector in order to generate normalized images from the solid-state X-Ray detectors. There is also a need in the art to identify bad pixels in the solid-state X-Ray detectors without projecting an X-Ray beam onto the detector. There is furthermore a need in the art to calibrate solid-state X-Ray detectors without interrupting productive use of the X-Ray system.